You are right that he should have been fired earlier; but we aren't privy to the internal politics of the company. Perhaps there was/were/are lawyers involved in this.. we don't know.

While it's just Twitter, he specifically name dropped both the GM and Studio Director of Bioware, thanking them for letting him "speak on things". While there very well could have been some internal stuff going on (he apparently no longer works for Bioware), they essentially didn't do a damn thing about it. Even if they did fire him, they clearly waited until the game was complete to make a move that should have been made two years ago.